Esthesiometry (es-the-si-om-e-try) is the measurement of sensation, specifically tactile. The measurement of corneal sensation evaluates the ophthalmic branch of the fifth cranial nerve (trigeminal). An esthesiometer or aesthesiometer is a device used to measure sensation. To test for corneal sensation there are qualitative and quantitative methods. The most commonly used method in clinical practice which is qualitative in nature, is the use of a cotton-tipped applicator. Topical anesthetics should not be used prior to testing corneal sensation.
The first esthesiometer was described in 1894 by von Frey and was built using horse hairs of different lengths. In 1932 Francheschetti, improved on von Frey’s version and then in 1956 Boberg-Ans described a device using a single nylon thread with a constant diameter but variable length. Cochet-Bonnet improved on the Boberg-Ans version and developed two different models. One model uses a diameter of 0.08 mm which allows pressure of 2 to 90 mg/0.005 mm2 and the second model uses a diameter of 0.12 mm with pressure ranging from 11 to 200mg/0.0113 mm2
Corneal esthesiometry is typically used clinically to evaluate for neurotrophic keratopathy. In research, esthesiometry has been used for various purposes to include recording the duration of an analgesic on the cornea or indicating the corneal health in long-term contact wearers.
The qualitative method is most commonly used in clinic and often achieved with a cotton-tipped applicator because it is easily accessible. Alternatively, a small piece of dental floss may be used. No topical anesthetics should be used prior to performing the test. A wisp of the cotton-tipped applicator is used to compare sensation in each eye. It is recommended to approach the patient from the side and test all four quadrants. Record the sensation in each location as normal, reduced, or absent.
There are various quantitative methods that are typically reserved for research or complicated cases. The most common quantitative method is the handheld esthesiometer (Cochet-Bonnet). Other methods reported include:
- Non-contact air puff technique
- Chemical stimulation using capsaicin
- Thermal stimulation with a carbon dioxide laser
Handheld esthesiometer (Cochet-Bonnet)
The handheld esthesiometer (Cochet-Bonnet) is a device that contains a thin, retractable, nylon monofilament that extends up to 6 cm in length. Variable pressure can be applied by the device by adjusting the length. The monofilament ranges from 60 mm to 5 mm and as the length is decreased the pressure increases from 11 mm/gm to 200 mm/gm. The Luneau Cochet-Bonnet Aesthesiometer is sold by Western Ophthalmics Corporation© for $500.
Steps for using the handheld esthesiometer:
- Extend the filament to full length of 6 cm
- Retract the filament incrementally in 0.5 cm steps until the patient can feel its contact
- Record the length (NOTE: The shorter the length indicates decreased sensation.)
- Compare the fellow cornea
- Repeat steps 1-4 in each quadrant: superior, temporal, inferior, nasal
- Sterilize the filament and retract back into the device to protect it from damage
The ophthalmic branch of the fifth cranial nerve ("CN V") carries sensory fibers for the cornea. Key points to note about ocular sensation include:
- Greatest in the central cornea except in elderly patients where it is more sensitive in the periphery
- Drops rapidly as distance increases from the limbus
- Falls with increasing age
- Is not affected by iris color
- More sensitive in the temporal limbus than the inferior limbus
- Reduction has been reported in Diabetes Type 1 and Type 2
Corneal Hypoesthesia Differential Diagnosis
Corneal hypoesthesia can occur from any etiology that causes CN V damage. Important etiologies to consider include:
- Herpes simplex keratitis
- Herpes zoster ophthalmicus
- Surgical trauma (PK, LASIK, large limbal incisions, ablation of the trigeminal ganglion)
- Topical medications (anesthetics, NSAIDs, ß-blockers, and carbonic anhydrase inhibitors)
- Cocaine abuse
- Cerebrovascular events
- Tumors (acoustic neuroma, neurofibroma, or angioma)
- Multiple sclerosis
- Hansen disease (leprosy)
- Hereditary causes - Familial dysautonomia (Riley-Day syndrome)
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- Salinger, C, "Neurotrophic Keratitis: What's in the Toolbox," Ophthalmology Management, June 1, 2020.
- External Disease and Cornea, Section 8. Basic and Clinical Science Course, AAO, 2010.
- Western Ophthalmics Corporation. Luneau Cochet-Bonnet Aeshesiometer. http://west-op.com/aesthesiometer.html. Accessed 01 DEC 2011.